Phase-shift-keying for the transmission of binary or other signals has been shown to have advantages over other modulation techniques including a lower error rate in the presence of Gaussian noise. Its use in practice has been limited owing to the problems associated with the regeneration of the carrier at the point of reception and problems with phase-locked loop based demodulators. The latter problem has been resolved somewhat using a tan-locked loop. The former problem is related to the use of a 180 degree phase-shift for modulation to ensure waveform continuity in transmission.
Digitally controlled phase shifters usually rely on a P.L.L. technique in which various harmonics are generated. This allows phase-shifts to have values of m360/n degrees to be implemented (m is an integer and n is the harmonic number). While such techniques are adequate for P.S.K. generation, the frequency range of operation is limited by the lock-in range of the P.L.L. and the phase steps are large and discrete. It is more difficult to obtain values for n greater than 8 which results in a comparatively large modulation bandwidth.
A prime requirement of a P.S.K. demodulator is the regeneration of the carrier so that phase changes can be recorded without loss or ambiguity. This is particularly important when the P.S.K. transmission involves phase-shifts of 180 degrees. For any other phase-shift, one can make a decision about whether the shifted phase leads or lags the carrier from the magnitude of the phase-shift.